Neuronal Physiology Core Abstract: The studies proposed in this Program Project will define the structure, function and regulation of hindbrain neural circuitry that underlies gut-brain communication and ascertain alterations that occur within this circuitry in response to bariatric surgery. To probe the function and regulation of distinct neuronal populations within this circuitry, at the cellular level, the Neuronal Physiology Core will conduct specialized assays of cellular function that are targeted to select neuronal subtypes using genetically encoded tools combined with unique genetic mouse models. These assays include 1. in vivo fiber photometry to measure calcium signals from genetically encoded calcium indicator (GECI)-expressing hindbrain neuronal populations, as a readout of cellular activity in response to stimuli, pre and post-VSG. 2. ex vivo electrophysiologial analysis of genetically-labeled and indentified neurons to directly assess cellular mechanisms underlying function and regulation pre and post vertical sleeve gastrectomy (VSG) 3.combined optogenetics and ex vivo electrophysiology to assess the functional connectivity of defined neuronal populations 4. in vivo optogenetic stimulation or inhibition of distinct genetically targeted neuronal populations to assess the impact of their modulation on physiologic processes (e.g. feeding behavior, conditioned place preference)